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As far as we publicly know, the iPhone 5 will use a maximum of 5W (5V x 1.0A) to charge. The maximum power for for micro USB by specification is 9W (5V x 1.8A). Even if using the 12W charger (or a 1000W charger), the iPhone will only draw 5W. If you were to use another device that wanted more power (iPad 4th gen - wants up to 12W), you might be limited by the physical USB connector cable as 9W might be the maximum physically possible. If more power was delivered through the micro USB cable, it may heat up, cause a fire, or short circuit unless the cable and pins were intentionally designed to surpass specification.

You shouldn't be using a micro-USB cable with an iPhone 5. iPhone 5 uses regular USB to flash connector cables (they come with one). These cables will use 12W no problem if the device can handle 12W. iPads using flash connectors will also use as much power as the need. If the iPad needs 12W, and the adaptor provides 12W, it can use 12W with the standard USB to flash cable that comes with your iPhone 5 or recent iPad. Limitations of micro-USB to 9W do not apply here using Apple cables because they are not micro-USB standard. They are built to Apple's flash connector specs and do not have a micro-USB connector on them.